Pressure maintenance and repressuring in oil and gas fields



Filed April 8, 1953 NOV. 4, MOLL ET AL PRESSURE MAINTENANCE AND REPRESSURING IN OIL AND GAS FIELDS FIG] INVENTORS BERNHARD MOLL, OTTO KRIEGBA UM (My-01 maq flq' ATTORNEYS 2 Sheets-Sheet 1 NOV. 4, 1958 B, MQLL ET AL 2,858,891

PRESSURE MAINTENANCE AND REPRESSURING IN OIL AND GAS FIELDS Filed April 8, 1953 25heets-Sheet 2 FIG.2 6

J T 16 i Q INVENTORS BERNHARD MOLL, 0T7UKR/EGBAUM BY I v "\M AT TORNEYJ rnnssnnn MAINTENANQE AND REPRESSURTNG IN on. AND GAS FIELDS Application April 8, 1953, Serial No. 347,499 Claims priority, application Germany May 16, 1952 7 Claims (Cl. 166-11) The present invention relates to a process of maintaining and restoring the pressure in oil and gas fields.

Petroleum as occurring naturally in the earth is commonly associated with natural gas. Reservoir rocks containing natural gas only generally overlie the oil in the productive formation. When the natural gas is under high pressure it provides the energy'for the expulsion of the oil from the reservoir without pumping being required. The oil thus produced is mostly associated with portions of the gas which are simultaneously lifted to the surface. The production of oil by this process leads to a gradual decline in pressure in the reservoir. Gas pressure, however, is required to force the oil to the surface by natural flow and is necessary when an oilfield is exploited by pumping since it is beneficial to driving the crude oil towards producing wells. Extensive experiments and efforts, therefore, have been made to maintain the gas pressure for as long a time as possible and to restore the original pressure after the latter has decreased. A frequently adopted method consists in injecting, in the vicinity of producing wells, natural gas under pressure into the oil pool or the overlying gas bearing stratum. The natural gas may be taken from distant reservoirs of from the producing wells. At any rate, the gas must be deducted from the produced quantity of natural gas intended for commercial application. Even though some of this gas is returned to the surface afterwards, the expense is so excessive as to seriously impair the commercial value of the process.

It is already known to maintain and restore the pressure in oil and gas pools by supplying air under pressure. This process, however, entails the great danger of fire and explosion when the air is contacted with the natural gas contained in the producing wells.

The principal object of the invention is to eliminate the disadvantages and hazards associated with the previously known methods and to provide a novel and economic process for maintaining and restoring the pressure in oil and gas fields. Other objects will become apparent as shown by the following description and drawings wherein:

Fig. 1 is a somewhat diagrammatic view of one embodiment of the invention; and a Fig. 2 is a somewhat diagrammatic view of a second embodiment of the invention.

In accordance with the present invention we have found that pressure maintenance and repressuring in oil and gas fields can be achieved by supplying an inert gas to the oil and/or gas reservoirs. The inert gas entering and penetrating the producing formation pneumatically forces the petroleum to the surface. The term inert gas" is used throughout the specification and claims to denote gases which are free or substantially free from oxygen and therefore non-reactive with natural gas so that any fire and explosion hazards are safely avoided. Examples of inert ases are flue gases and other combustion or States Patent exhaust gases. Said inert gases are readily available in territories Where oil occurs and, apart from the compression costs which accrue at all events, no expenses are involved by the use of the above-said inert gases. Nitrogen or steam may be also advantageously employed as inert gas.

It is further within the scope of the present invention to maintain and restore thepressure in gas and oil fields by supplying compressed gases containing oxygen, for instance air, or fluids splitting off oxygen such as hydrogen peroxides, and solid peroxides such as sodium peroxide into the reservoir rock under carefully controlled conditions so as to eliminate any fire and explosion risk.

By drilling a bore hole into a gas reservoir, in the well known manner, the well shaft, after being tightly sealed with a top plate, is fill-ed up with natural gas under high pressure, say 60 atmospheres. Before supplying compressed air into the well containing natural gas, which step would certainly constitute a great fire and explosion risk, the well is cleaned of natural gas by injecting an inert gas under a pressure higher than that of the natural gas, say atmospheres. The gas in the bore hole is repeatedly expanded and compressed until the well is substantially filled with a gas which is free from combustible constituents. Thereupon compressed air is injected into the well to enter and to penetrate the reservoir rock, thus maintaining or restoring the pressure in the formation in a manner which will be more fully explained in the following.

A specific and particularly advantageous method of cleaning wells from natural gas consists in putting into a well casing a second tube extending to the perforated end of the casing, and injecting through this tube the inert gas while, at the same time, the mixture formed from natural gas and inert gas is vented through the top of the well until it no longer contains any combustible constituents.

A typical arrangement embodying the afore described cleaning operation is illustrated by way of example in Fig. 1 of the accompanying diagrammatic drawings, in which:

Numeral 1 designates the well casing which is perforated at its lower end 19 and tightly sealed with a top plate 2. The casing 1 is provided on one side with ashutoff device (a slide valve) 6, sealingly extends through an opening in plate 2 to the lower end of the casing 1. An inert gas is supplied through the shutoff device 6; and this injected inert gas leaves the tube 5 at its lower end, flows partly into the surrounding gas reservoir and partly in an upward movement within the space formed by the tube 5 and the casing 1 and is released through the pipe 3 and the shutofi device 4 until the issuing gas no longer contains any combustible constituents. Compressed air is then injected through the shutofi devices 4 and/ or 6.

A well having an inside diameter of 100 mm. and a depth of 1000 mm. contains at a pressure of 60 atmospheres about 500 cbm. of gas. This means that a limited quantity only of inert gas is required to clean a well by the herein described process.

As already mentioned before, inert gases are inexpensive and readily available in territories where oil is produced, being supplied from the exhaust gases of the internal combustion engine driving the compressor for the gases to be injected into the reservoir rock. After cooling and cleaning the exhaust gases, they are passed to the suction pipe of the compressor. At high pressures the energy of the driving engine (diesel, gasoline or gas engine) is insufficient to compress the total quantity of the exhaust gases. A part of the gases, therefore, is allowed to escape or, if electric energy is available, an electric motor generating the additional energy is mounted on the shaft driving the internal combustion engine and the compressor. By injecting compressed air after cleaning the wellfrom'natural gasby means of an inert gas, the latter is passed into the 'gas bearing stratum and," at the'beginning, pushes back and follows the natural gas into more distant regions ofthe reservoir rock. After some time, however, the'natural'gaswill mix with the air in the formation at some distance from the well. Assuming that the gas reservoir rock is relatively'porous so that it has, for example, a porosity of 25percent, 0.25 liter of the mixture of gas and air can form in the required ratio of 1:9.52 at, for example, 60 atmospheres and 30 C. This air-gas mixture corresponds'to about 0.016 to'0.0l7 Nm. or 0.00152 to 0.00161 Nm. of natural gas, the remainder being air. In the combustion of 1.5 to 1.6 liters of natural gas per 1 liter of rock, 13 kilo'calories are released. The reservoir rock generallycomp'rises sand of which 1 liter'weighs 1.5 kg. The 'specific'heat of sand is about 0.22 k.cal./kg.; 0.33 kjcal. areth'us required to raise the temperature of one liter of' sand one degree centrigrade. Hence it follows that the 13' kilocalories generated in the combustion of the gas content of'o'ne liter of sand will raise the temperature of the sand only about 40 C. to 70 C. This means the sand is maintained within a temperature range wherein a'cornbustion, if such had been initiated at all by a fortuitous concurrence of circumstances, will stop by itself since the ignition temperature of methane in the air at normal pressure is 700 C. Furthermore, it should be noted'that' the generated combustion gas must be raised the same interval in temperature. This means that the amount of heat required in the above example in order to heat thesand is actually not even available. It is apparent therefore that'any'accidental combustion in the gas reservoir rock will inevitably stop at once.

Obviously there is the danger that, in the course of time, the injected air will approach and'reach anadjacent well and is lifted therein to the surface after admixing with natural gas, possibly in the form of mfex msive mixture. Anyhazards which may be involved thereby'are safely avoided by stopping the' production of this well as soon as the gas analysis shows the first traces 'of oxygen, and then supplying air under pressure in the well in the herein described manner, i. e. after cleaning the well from natural gas, and continuing the exploitation from a third well. It may be presumed that, in the course of time, the oxygen reacts with the natural gas under the pressures prevailing in a reservoir rock and is gradually reduced; however, as long as no evidenceof such reaction can be found, it should be advisable to conduct the process so that the oxygen of the air burns with the natural gas or the crude oil contained in the formation. By this process the underground is heated up, thus increasing the fluidity of the oil which is beneficial to the flow towards the well. 7

A continuous combustion in the formation can be effected only by preheating the injected air ,to suchde gree that the ignition temperature is reached and nedasflame f m-.a a es.-.., he fl w ng a is reh a ed by -.and. i ltah' g a u b a edwel fe at a; The-.- rthe p ated m st h n b sea eed.

with fresh natural gas to.effect combustion. Thus it is necessary to create in the forrn e-wi h the qui dh hig ion te ,erature. ,This

ionfabh lme was 1 is achieved fonexam le .by. the arrangeinentjshown'in Fig; Z of the accompanying drawings,.;in whigh the casm'g 1 i -per a ed its lowe P 1,9; i re ati-W1Y me H 2 andprovided on one s1de,with aashort p pefi that, can.

betclosed with a shutoff device, (slide .valvej-fi. Theftub e 10 provided onboth sideswith'the twoysho rt. pipe v 1 1. and--12; which cambe closedby-means of the shutoff 'devices-1 13 and 14-.respectively, extends downwardsthrough the top plate 2 of the tubular casing 1 and terminates adjacent the perforated part 19. The tube 10 is sealingly closed by a top plate 20 and s'ealingly passed through the top plate 2 of the casing 1. Another tube 5, which is provided with a shutoff device 6, is sealingly led through the top plate of the tube 10. A temperature gauge 16 leading to the surface is provided at the end of the tube 10. An igniter 17, for instance an incandescent filament, which can be operated on the surface, is stretched over the opening of the lower part of the tubes 10 and 5. When the shutoff devices 4, 13, 14 and 6 are closed, the well is completely filled with natural gas. By feeding compressed inert gas through the short pipe 12 and opening the shutofl. devices 14 and 4, the natural gas is displaced from the interior of the tube 10; a part of the inert gas passing into the formation about the well, with another part moving upwards in the casing 1 and issuing through the open shutofi device (valve) 4. When the gas analyses show that the gas escaping through the pipe 3 no longer contains any combustible constituents, the supply of inert gas is stopped and air under pressure is injected into the tubing 10 through the pipe 12; at the same time, the igniter 17 is contacted and a compressed liquid or gaseous fuel, for instance natural gas and gas oil, is injected through the tube 5. The injected air and the fuel gas mix with one another at the lower end of the tubes 10 and 5 and are ignited by the igniter 17. The flue gases thereby generated are initially caused to flow inwardly in the casing 1 and issue through the pipe 3. During the uplift, the flue gases transfer part of their heat to the tube 10 wherein air flows downwards. This air is heated up and the temperature of the flame at the end of the tubes 10 and 5 increases. By closing the shutoff device 4 the pressure is controlled so that part of the flue gases penetrates and heats thesurrounding formation. The pipe'3, therefore, is provided with a gauge disc 18 to measure the quantities of the escaping flue gas as well as the quantities of the supplied fuel gas and air. The thermocouple element 16 permits a control of the temperature of the flame. The temperature can be prevented from rising t'oo high'by admitting cooling water through the short pipe 11. The cooling can also be effected by the addition of an inert gas or by throttling back the fuel gas.

The flame is allowed to burn for a prolonged period. After a time, the gas'issuance through the pipe 3 is gradually reduced and the admission of air through the pipe 12 is increased in such' a manner that the flue gas which nowpenetrates the formation in relatively large quantities contains a certainamount of oxygen so as to allow the combustion of further quantities of gas and oil in the reservoir rock. By carrying on this process for a sufficiently long time, the formation about the well reaches a temperature which ensures the continuance of the combustion, once initiated, without further control, as long as the gas supplied under pressure contains oxygen. It is possible to calculate from the burnt gas and the supplied air in proportion to the released flue gas, the apthrough 'thetube s and the release of flue gas through the pipefd is then stopped. By connectingthe pipe S'to an air' supply pipe, compressedairis admitt'edto' the casing 1 to prevent the casing 1 from" being overheatedby 'the fire which may also possiblybe burning outside the 'casing 1. To avoid any troublej thelower parts 'ofthe casing lland'the tubes 1 0 atlas are' prefe'rably made; of refractory material.

a matter of fact, ehang es'rnay be made-in the':operationof the arrangement as *des'cribed'in' the foregoing; for

instance by siipplying'aninert g'a's'and, subsequently, air' into the tube5'and injecting the fuel gas through the tube-10.'-

Onceacornbustion has properly been initiated it will continue as long as' airis admitted. Since natural gas is of greater weight than air, it is advantageous to initiate the combustion in the lower levels of the gas reservoir rock so that the rising air meets with natural gas. If the desired temperatures are not obtained in the formation, the air for combustion can be enriched with oxygen, for instance by addition of pure oxygen. If the fire is to be extinguished, the supply of air is stopped and the well is again cleaned withinert gas, as at the beginning of ignition. If the gas escaping through pipe 3 no longer contains oxygen, the well may be used for lifting natural gas. It will be advisable, however, to continue the control of the gas composition for a prolonged time and, if necessary, to inject further quantities of inert gas.

Since the combustion gases and the natural gas are pushed back by the following compressed air injected into the formation there is a possibility that, after some time, the air, when contacting the natural gas, will no longer be hot enough; under these circumstances, the combustion might cease. This hazard can be avoided by allowing the combustion to proceed in the oil bearing stratum since a fire once initiated there continues to burn. Since the generated flue gases do not displace the oil, nor perhaps the low boiling constituents, under these conditions the combustion once started will not stop. The combustion is initiated in accordance with the process described with respect to gas; however, the required ignition temperature in the oil reservoir rock is probably reached sooner as a larger quantity of fuel is available per liter of rock than in the event of the gas bearing stratum. Also in this case the combustion temperature can be lowered by introducing water or can be raised by supplying oxygen. Oxygen may also be additionally supplied to the formation in the form of oxygen releasing fluids such as hydrogen peroxide. Solid peroxides such as sodium peroxide may also be used. 1

As a matter of fact, no producing well should be selected for this purpose unless it is completely or almost completely exhausted. The reservoir rock of such wells still contains a considerable portion of petroleum since it is not possible to extract the oil content from a reservoir rock completely. Such residual quantity suflices, however, to bank up a combustion once started.

At the beginning of the combustion the well will be almost dry, in other words, free from oil. The combustion initiated by the additionally supplied gas results in a heating of the formation surrounding the casing 1; consequently, the oil in the reservoir rock becomes more fluid and no longer adheres to the formation but flows into the lower part of the bore hole. Arrangements have to be 'made that this oil is continually lifted. This can be done for instance by sinking the two tubes 10 and 5 to the deepest point of the bore hole after the combustion has been initiated. By feeding an inert gas into the inner tube 5 a gas lift carrying the oil from the deepest point to the surface can be applied in known manner.

Obviously many modifications and variations of the invention as described above may be made without departing from the spirit and scope thereof, and thus only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A process for maintaining the pressure in hydrocarbon deposits surrounding a well which comprises introducing a stream of cold inert gas under pressure into the well to drive the naturally-occurring hydrocarbon gases to the surface, maintaining the flow of the stream of inert gas until the interior of the well is substantially free of said hydrocarbon gases, thereafter introducing a combustible gas and an oxygen-containing gas into said well and mixing them together, igniting said mixture to form a combustion gas, at least a portion of said combustion gas, while passing to the surface of the well, acting to pre-heat additionally entering oxygen-containing gas, said pre-heated oxygen-containing gas heating the interior of the well and the surrounding deposit formation to the point where ignition of the oil in the formation can take place, then increasing the pressure of the incoming mixture of combustible and oxygen-containing gases by throttling the flow of combustion gas to the surface while increasing the content of oxygen in the mixture, whereby the heated, oxygen-enriched combustion gas is forced into the surrounding deposits to ignite the oil therein, and then halting both the flow of combustion gas to the surface and the introduction of combustible gas while maintaining the flow of the oxygen-containing gas to effect a continuous combustion within the deposits.

2. The process of claim 1 wherein the compressed oxygen-containing gas is pre-heated before being intermixed with said combustible gas.

3. The process of claim 1 wherein said oxygen-containing gas is air.

4. The process of claim 1 wherein said oxygen-containing gas is substantially pure oxygen.

5. The process of claim 1 wherein said oxygen-containing gas is generated by oxygen-releasing compounds.

6. The process of claim 1 wherein coolant water is introduced, after a predetermined temperature is reached.

7. The process of claim 1 wherein the combustion is controlled by regulating the relative amounts of the oxygen-containing gas and the combustible gas introduced into the Well.

References Cited in the file of this patent UNITED STATES PATENTS 968,851 Illy Aug. 30, 1910 1,249,232 Squires Dec. 4, 1917 1,457,479 Wolco'tt June 5, 1923 2,173,556 Hixon Sept. 19, 1939 2,390,770 Barton et al Dec. 11, 1945 2,421,528 Steffen June 3, 1947 2,444,755 Steffen July 6, 1948 2,584,606 Merriam et a1 Feb. 5, 1952 2,642,943 Smith et a1. June 23, 1953 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,858,891 November A, 1958 Bernhard Moll. et aln It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 39 for "reservoirs of" read m reservoirs or vcolumn 2, 1ine 59', for "1000 mm," read e: 1000 m,

Signed and sealed this 16th day of June 1959.

SEAL) Attest:

KARL H. AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents 

1. A PROCESS FOR MAINTAINING THE PRESSURE IN HYDROCARBON DEPOSITS SURROUNDING A WELL WHICH COMPRISES IN- 